1. Field of the Invention
The present invention concerns a method for manufacturing monocrystalline layers of materials with different lattice parameters.
In general, to make semiconductor devices, at least one monocrystalline layer is grown epitaxially on a substrate. Now this epitaxial growth of a material on a substrate is very difficult when the mismatching of the lattice parameters is very great. However, for reasons of cost and specific technical characteristics, it is ever increasingly being sought to make substrates of silicon or other materials of the same type in the manufacture of opto-electronic devices such as lasers, photodiodes, photocathodes or similar devices and microwave devices such as GUNN diodes, and IMPATT diodes, FETs which are made with III/V compounds of the periodic table of elements, or infrared detectors made with multiple quantum well structures of III-V materials, or by means of II/VI or IV/VI compounds of the periodic table of elements. Thus, the mismatching of the lattice parameters, given by the ratio .delta.a/a, is 4.1% between gallium arsenide and silicon, and 8.1% between indium phosphide and silicon. As a consequence, dislocations are created during growth. In effect, since the energy per unit of interface between the two materials increases proportionally with the thickness of the layer, for a given critical thickness it becomes more advantageous, in terms of energy, to create a dislocation than to continue the epitaxy under anisotropic biaxial compressive stress. Now, a number of dislocations cross the layer. This is a very great hindrance to the efficient working of the devices made on these epitaxiated layers. Thus, in the case of an epitaxy of GaAs on silicon, 10.sup.12 dislocations per cm.sup.2 at least are not localized in the vicinity of the interface.
2. Description of the Prior Art
At present, there are different methods aimed at reducing the number of dislocations in the upper part of the layer. Among these methods, we might cite the use of thermal annealing cycles, done off site. This thermal annealing modifies the direction of the dislocation lines without modifying their number. Another method uses in situ annealing cycles which considerably improve the rate of dislocations if they are made after about 1000 .ANG. of growth at low temperature without, however, reducing them beyond the critical value. Other methods include the use of superlattices between the substrate and the monocrystalline layer, the use of quantum well structures, the efficiency of which is limited when the rate of dislocations is very high or the use of cycles of growth which may or may not be coupled with annealing cycles. However, none of these methods has made it possible to reduce the rate of dislocations to below a value of 10.sup.6 per cm.sup.2.